Manufacture of selenium elements



Feb. 27, 1945. o. SASLAW 2,370,493

MANUFACTURE OF SELENIUM ELEMENT Filed May 1, 1943 IN VEN TOR. 0 770 67/514 W ATTORNEY Patented Feb. 27, 1945 UNITED STATES. PATENT OFFICE MANUFACTURE OF SELENIUM ELEMENTS Otto Saslaw, Lyndhurst, N. J., assignor to Federal Telephone & Radio Corporation, New York, N. Y., a corporation of Delaware Application May 1, 1943, Serial No. 485,381 7 Claims. (Cl. 175-366) This invention relates to selenium elements and has for its object to improve the qualities and characteristics of such elements.

Selenium elements such as rectifiers are commonly made by placing an adherent layer of selenium on a carrier plate and heat treating the selenium to crystallize it to the metallic form. A counterelectrode is then placed over the metallic selenium surface, for example by spraying on a suitable alloy metal which closely adheres to the selenium. -It is then a common practice to electro-form the element by connecting it in a D. C. circuit with the counterelectrode connected to one electrical terminal and the carrier plate to the other. Such an element exhibits the well-known rectifying characteristics of transmitting much more current in one direction than in the other when placed in an alternating current circuit. It is a common theory that the rectifying action is due to the presence of a blocking layer of minute thickness which forms at the junction surface between the selenium layer and the counterelectrode over it.

In accordance with my invention I improve the rectifying characteristic of the element by applying to the selenium a small amount of water, preferably distilled water, before the application of the counterelectrode. This is preferably done during the heat treatment. It is the common practice to subject the selenium surface to pressure during the heat treatment and to use a pressure disc or plate in contact with the selenium surface of a material which does not readily react with .or contaminate the selenium, for example, a mica disc. I prefer to apply the water in the form of a drop on the pressure plate, then place the pressure plate with the Water drop against the selenium surface and then subject the element to pressure during the heat treatment. By such a treatment the water apparently permeates the selenium and improves its properties so that after the counterelectrode is applied and the element electro-formed, it has a much higher resistance to current in the reverse direction and forms up to a higher voltage than otherwise.

Selenium elements have heretofore been treated with humidity but these have been surface treatments, for example by placing the selenium coated disc after conversion to the metallic form in an atmosphere of water vapor or,moisture which puts the moisture on the surface. I have found that my treatment, however, wherein the drop of water is applied during the heat pressure treatment causes the water to saturate the selenium producing a different and better effect than by a mere surface treatment with moisture.

The invention will be better understood from the following detailed description considered in conjunction with the accompanying drawing of which:

Fig. 1 illustrates a selenium coated element to which the water treatment may be applied;

Fig. 2 illustrates a pressure disc containing a drop of water; and

Fig. 3 shows in perspective the arrangement of the selenium coated element and pressure disc between the platens of a press.

The selenium element shown in Fig. 1 comprises a base plate I which may, for example, be of aluminum, iron or steel or may be nickel plated if desired. In accordance with a common practice it is shown with a central hole 2 although it will be understood that the central hole need not necessarily be used. The surface of the plate is commonly roughened and there is then applied to it in a well-known manner a layer 3 of selenium. This may, for example, be applied by melting selenium powder and spreading it over the surface of the plate, then allowing the selenium to cool, leaving an adherent layer of vitreous selenium. Before this is useful in a selenium cell it needs to be crystallized to the metallic form and this is commonly done by heat treating it to a temperature somewhat below the melting point of selenium; and pressure is ordinarily applied to the selenium surface during some or all of the heat treatment.

To improve the rectifying properties according to my invention I place a drop or so of water, preferably distilled, on the surface of the pressure sheet 4 used as a pressure disc against the selenium surface. Fig. 3 illustrates an arrangement showing the pressure sheet 4 which may be, for example, mica having the drop of water 5 on its under surface and facing the selenium layer 3. The selenium element is then compressed with the pressure sheet against the selenium surface between the platens 6 and 1 of the press, and preferably heat is applied during the pressure treatment.

A satisfactory pressure treatment is to apply heat at a temperature of around to C. for a time such as a half hour or so, after which the selenium element may be taken out of the press and away from the pressure sheet 4 and the heat treatment continued without an pressure at a higher temperature of, for example, around 200 C. During the time the pressure sheet with its drop of water was in contact with the selenium, the water has had a'chance to permeate the selenium and perform its beneficial function. f e

After the heat treatment is ended-the counterelectrode may be applied, for example, by spraying with Wood's metal or the like over the treated selenium surface, .after which disc may be electro-formed-in the usliaHn er.

I have found that a selenium element thus treated with water has a better rectificationratio than one which is not so treated, as its resistance in the reverse current direction is higher while its conductance in the forward direction is not reduced.

What is claimed is:

1. The method of treating a selenium coated 7 element which comprises compressing the element at an elevated temperature with a small quantity of water in contact with the selenium.

2. The method of treating a selenium element which comprises compressing it at an elevated temperature in contact with a pressure element having a small amountof water between the pressure element and theselenium.

3. The method of treating a selenium coatedelement which comprises'compressing the element at a temperature in'th'e neighborhood of 100 to 120 C. with some water in contact with the selenium surface.

4. The method of treating a selenium coated element which comprises compressing the ele-' "ment at a temperature or around 100to 120C.

at an elevated temperature with a small quantity of water in contact with the selenium in vitreous form.

7. The method of treating a selenium coated element comprising a base plate carrying-a layer of selenium in vitreous form which comprises I applying a pressure plate to the selenium surface, inserting between the pressure plate and the selenium surface an amount of water not greater than that which can be carried by said surface, applying pressure through the pressure plate to the wet surface of the selenium and heating the selenium during the application of said pressure.

' OTTO SASLAW. 

